Modulus % in Django template

You need divisibleby, a built-in django filter.

{% for p in posts %}
    <div class="post width1 height2 column {% if forloop.counter0|divisibleby:4 %}first{% endif %}">
        <div class="preview">

        </div>
        <div class="overlay">

        </div>
        <h2>p.title</h2>
    </div>
{% endfor %}